Actuatable occupant restraint systems are well known in the art. Such systems include a crash sensor and an actuatable restraining device. When a crash condition is sensed by the crash sensor, the restraining device is actuated. One known type of crash sensor used in occupant restraint systems is an inertia sensor.
An inertia sensor typically includes a weight movably mounted in a housing so that the weight moves relative to the housing in response to the sensor being subjected to a change in force. When a vehicle is in a crash condition, the weight of an attached sensor moves relative to the sensor housing since the crash effectuates force changes. The sensor includes or is connected to an electric circuit which is arranged so that movement of the weight relative to the housing for a predetermined distance closes the electric circuit. Such movable weight inertia sensors are typically mounted to a vehicle so that the direction of relative movement between the weight and the sensor housing is parallel to the front-to-rear axis of the vehicle. In other words, the sensor's axis of sensitivity is parallel with the front-to-rear axis of the vehicle.
A vehicle crash into a barrier oriented 90.degree. relative to the front-to-rear axis of a vehicle is referred to in the art as a frontal crash. All other crashes are referred to as non-frontal crashes. A movable weight inertia sensor mounted parallel to the front-to-rear axis of a vehicle provides a "measurement" of the inertia of a frontal crash. If the vehicle is subjected to a non-frontal crash, the inertia sensor provides a "measurement" of a component of the inertia having a value functionally related to the sine of the angle between the direction of travel of the vehicle and a line normal to the object hit.
Other types of vehicle crash sensors measure "crash energy." Actuation of the occupant restraint system is controlled in response to the measured crash energy. Such energy type crash sensors are typically mounted to the vehicle so as to measure the energy of a frontal crash, i.e., the axis of sensitivity of the sensor is parallel with the front-to-rear axis of the vehicle. Similarly, if the crash is a non-frontal crash, the sensor provides an indication of a component of the crash energy along the sensitivity axis of the sensor.
Occupant restraint systems may include a plurality of restraining devices such as a plurality of airbags surrounding the occupant. Occupant restraint systems may also include a combination of different types of restraining devices such as a plurality of airbags and a plurality of seat belts having actuatable locking devices In such occupant restraint systems having a plurality of restraining devices, it is desirable to be able to determine the direction of the crash and actuate those devices which are located to best protect the occupant during the crash. It is further desirable to determine the magnitude of the crash so as to control timing of the actuation of the restraining device.